U.S. patent number 7,518,284 [Application Number 11/592,506] was granted by the patent office on 2009-04-14 for dielectric composite and a method of manufacturing a dielectric composite.
This patent grant is currently assigned to Danfoss A/S. Invention is credited to Mohamed Yahia Benslimane, Peter Gravesen.
United States Patent |
7,518,284 |
Benslimane , et al. |
April 14, 2009 |
Dielectric composite and a method of manufacturing a dielectric
composite
Abstract
A composite for a transducer facilitates an increased actuation
force as compared to similar prior art composites for transducers.
In accordance with the present invention, the composite also
facilitates increased compliance of the transducer in one direction
and an improved reaction time as compared to similar prior art
composites for transducers, as well as provides an increased
lifetime of the transducer in which it is applied.
Inventors: |
Benslimane; Mohamed Yahia
(Nordborg, DK), Gravesen; Peter (Nordborg,
DK) |
Assignee: |
Danfoss A/S (Nordborg,
DK)
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Family
ID: |
40453702 |
Appl.
No.: |
11/592,506 |
Filed: |
November 3, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090072658 A1 |
Mar 19, 2009 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10415631 |
Aug 12, 2003 |
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Foreign Application Priority Data
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Nov 2, 2000 [DE] |
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100 54 247 |
Oct 31, 2001 [DK] |
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PCT/DK01/00719 |
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Current U.S.
Class: |
310/311; 310/363;
310/367; 310/368; 310/800 |
Current CPC
Class: |
B81B
3/007 (20130101); H01L 41/083 (20130101); H01L
41/0836 (20130101); H01L 41/0986 (20130101); H01L
41/27 (20130101); H01L 41/333 (20130101); H01L
41/45 (20130101); H02N 1/006 (20130101); H04R
23/00 (20130101); H01L 41/0471 (20130101); B81B
2201/038 (20130101); Y10S 310/80 (20130101); Y10T
428/24612 (20150115) |
Current International
Class: |
H01L
41/08 (20060101) |
Field of
Search: |
;310/311,800,328 |
References Cited
[Referenced By]
U.S. Patent Documents
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WO |
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Other References
PCT Search Report for Serial No. PCT/DK03/00603 dated Feb. 5, 2004.
cited by other .
PCT Search Report for Serial No. PCT/DK03/00848 dated Mar. 25,
2004. cited by other .
Article entitled "Electrostrictive Polymer Artificial Muscle
Actuators" by R. Kornbluh, et al., SRI International, Proceedings
of the 1998 IEEE International Conference on Robotics &
Automation, Belgium, May 1998; pp. 2147-2154. cited by other .
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dielectrics for actuation" by Roy Kornbluh, et al., SRI
International; SPIE vol. 3669, pp. 149-161; Mar. 1999. cited by
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Trujillo, et al.; Presented at 2000 ASME International Mechanical
Engineering Congress and Exposition, Nov. 5-10, 2000; Orlando, FL;
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Article entitled "Spontaneous formation of ordered structures in
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cited by other .
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Periodicals, Inc.; pp. 1378-1383. cited by other.
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Primary Examiner: Budd; Mark
Attorney, Agent or Firm: McCormick, Paulding & Huber
LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of currently pending
U.S. application Ser. No. 10/415,631, filed Aug. 12, 2003, the
disclosure of which is incorporated by reference in its entirety,
and claims the benefit of and incorporates by reference in their
entirety essential subject matter disclosed in International
Application No. PCT/DK01/00719 filed Oct. 31, 2001 and German
Patent Application No. 100 54 247.6 filed on Nov. 2, 2000.
Claims
What is claimed is:
1. A composite comprising: a film made of a dielectric material and
having a first surface and second surface, at least the first
surface comprising a surface pattern of raised and depressed
surface portions, and a first electrically conductive layer being
deposited onto the surface pattern, the electrically conductive
layer having a corrugated shape which is formed by the surface
pattern of the film; wherein the second surface is substantially
flat.
2. The composite according to claim 1, wherein the electrically
conductive layer has a modulus of elasticity being higher than a
modulus of elasticity of the film.
3. The composite according to claim 1, provided as a web of
potentially unlimited length.
4. The composite according to claim 1, wherein the dielectric
material is a polymer material.
5. The composite according to claim 1, wherein the dielectric
material has properties similar to an elastomer.
6. The composite according to claim 1, wherein the film has a
largest thickness which is less than 110 percent of an average
thickness of the film.
7. The composite according to claim 1, wherein the film has a
smallest thickness which is at least 90 percent of an average
thickness of the film.
8. The composite according to claim 1, wherein the first
electrically conductive layer has a largest thickness which is less
than 110 percent of an average thickness of the first electrically
conductive layer.
9. The composite according to claim 1, wherein the first
electrically conductive layer has a smallest thickness which is at
least 90 percent of an average thickness of the first electrically
conductive layer.
10. The composite according to claim 1, wherein the surface pattern
comprises waves forming troughs and crests extending in essentially
one common direction, each wave defining a height being a shortest
distance between a crest and neighbouring troughs.
11. The composite according to claim 10, wherein the waves have a
shape which is periodically repeated.
12. The composite according to claim 10, wherein an average height
of the waves is between 1/3 and 20 .mu.m.
13. The composite according to claim 12, wherein each wave defines,
a largest wave having a height of at most 110 percent of the
average wave height.
14. The composite according claim 12, wherein each wave defines a
smallest wave having a height of at least 90 percent of the average
wave height.
15. The composite according to claim 10, wherein the film has an
average thickness being between 10 and 200 .mu.m.
16. The composite according to claim 10, wherein a ratio between an
average height of the waves and an average thickness of the film is
between 1/50 and 1/2.
17. The composite according to claim 10, wherein the waves have a
wavelength defined as the shortest distance between two crests, and
wherein a ratio between an average height of the waves and an
average wavelength is between 1/30 and 2.
18. The composite according to claim 10, wherein a ratio between an
average thickness of the first electrically conductive layer and an
average height of the waves is between 1/1000 and 1/50.
19. The composite according to claim 1, wherein the first
electrically conductive layer has a thickness in the range of
0.01-0.1 .mu.m.
20. The composite according to claim 1 and being substantially
(i.e. at least 10 times) longer in a lengthwise direction than in a
perpendicular crosswise direction.
21. The composite according to claim 20, wherein the surface
pattern defines a compliance direction in which the composite is
mostly compliant, wherein the compliance direction forms an angle
between 0 and 90 degrees to the lengthwise direction.
22. The composite according to claim 21, wherein the composite has
a compliance in the compliance direction which is at least 50 times
larger than its compliance in a direction being at least
substantially perpendicular to the compliance direction.
23. The composite according to claim 20, wherein the surface
pattern forms wave crests and troughs extending essentially in the
lengthwise direction.
24. The composite according to claim 20, wherein the surface
pattern forms wave crests and troughs extending essentially in the
crosswise direction.
25. The composite according to claim 1, wherein the surface pattern
comprises a plurality of identical sub patterns.
26. The composite according to claim 1, wherein the film has a
surface with a shape which is essentially unaffected by its contact
with the first electrically conductive layer.
27. The composite according to claim 26, wherein the composite has
a shape which is essentially unaffected by an elastic modulus of
the first electrically conductive layer.
28. The composite according to claim 26, wherein the film has a
surface with a shape which is essentially unaffected by the
thickness of the first electrically conductive layer.
29. The composite according to claim 1, wherein the film has a
surface with a shape which is essentially unaffected by an elastic
modulus of the film.
30. The composite according to claim 29, wherein the film has a
surface with a shape which is essentially unaffected by the
thickness of the film.
31. The composite according to claim 1, comprising a second
electrically conductive layer arranged on an opposite surface of
the film relative to the first electrically conductive layer to
form an electroactive composite.
32. The composite according to claim 1, wherein a resistivity of
the dielectric material is larger than 10.sup.10.OMEGA.cm.
33. The composite according to claim 1, wherein a resistivity of
the electrically conductive material is less than
10.sup.-4.OMEGA.cm.
34. A composite comprising: a film made of a dielectric material
and having a first surface and second surface, at least the first
surface comprising a surface pattern of raised and depressed
surface portions; and a first electrically conductive layer being
deposited onto the surface pattern, the electrically conductive
layer having a corrugated shape which is formed by the surface
pattern of the film; wherein the surface pattern forms wave crests
and troughs extending essentially in the lengthwise direction and
wherein the composite is substantially (i.e. at least 10 times)
longer in a lengthwise direction than in a perpendicular crosswise
direction.
Description
FIELD OF THE INVENTION
The present invention relates generally to the field of
micro-electro-mechanical systems (MEMS) and micromechanically
designed structures. More specifically, the present invention
relates to a micromechanical design of a mechanical structure which
substantially modifies its macromechanical properties. Even more
specifically, the invention relates to micromechanically shaping a
dielectric film to give it unique mechanical properties, which
reach through to the macroscopic mechanical properties of
transducers made by the film.
BACKGROUND OF THE INVENTION
An electrical potential difference between two electrodes located
on opposite surfaces of an elastomeric body generates an electric
field leading to a force of attraction. As a result, the distance
between the electrodes changes and the change leads to compression
of the elastomeric material which is thereby deformed. Due to
certain similarities with a muscle, an elastomer actuator is
sometimes referred to as an artificial muscle.
U.S. Pat. No. 6,376,971 discloses a compliant electrode which is
positioned in contact with a polymer in such a way, that when
applying a potential difference across the electrodes, the electric
field arising between the electrodes contracts the electrodes
against each other, thereby deflecting the polymer. Since the
electrodes are of a substantially rigid material, they must be made
textured in order to make them compliant.
The electrodes are described as having an `in the plane` or `out of
the plane` compliance. In U.S. Pat. No. 6,376,971 the out of the
plane compliant electrodes may be provided by stretching a polymer
more than it will normally be able to stretch during actuation and
a layer of stiff material is deposited on the stretched polymer
surface. For example, the stiff material may be a polymer that is
cured while the electroactive polymer is stretched. After curing,
the electroactive polymer is relaxed and the structure buckles to
provide a textured surface. The thickness of the stiff material may
be altered to provide texturing on any scale, including
submicrometer levels. Alternatively, textured surfaces may be
produced by reactive ion etching (RIE). By way of example, RIE may
be performed on a pre-strained polymer comprising silicon with an
RIE gas comprising 90 percent carbon tetrafluoride and 10 percent
oxygen to form a surface with wave troughs and crests of 4 to 5
micrometers in depth. As another alternative, the electrodes may be
adhered to a surface of the polymer. Electrodes adhering to the
polymer are preferably compliant and conform to the changing shape
of the polymer. Textured electrodes may provide compliance in more
than one direction. A rough textured electrode may provide
compliance in orthogonal planar directions.
Also in U.S. Pat. No. 6,376,971 there is disclosed a planar
compliant electrode being structured and providing one-directional
compliance, where metal traces are patterned in parallel lines over
a charge distribution layer, both of which cover an active area of
a polymer. The metal traces and charge distribution layer are
applied to opposite surfaces of the polymer. The charge
distribution layer facilitates distribution of charge between metal
traces and is compliant. As a result, the structured electrode
allows deflection in a compliant direction perpendicular to the
parallel metal traces. In general, the charge distribution layer
has a conductance greater than the electroactive polymer but less
than the metal traces.
The polymer may be pre-strained in one or more directions.
Pre-strain may be achieved by mechanically stretching a polymer in
one or more directions and fixing it to one or more solid members
(e.g., rigid plates) while strained. Another technique for
maintaining pre-strain includes the use of one or more stiffeners.
The stiffeners are long rigid structures placed on a polymer while
it is in a pre-strained state, e.g. while it is stretched. The
stiffeners maintain the pre-strain along their axis. The stiffeners
may be arranged in parallel or according to other configurations in
order to achieve directional compliance of the transducer.
Compliant electrodes disclosed in U.S. Pat. No. 6,376,971 may
comprise conductive grease, such as carbon grease or silver grease,
providing compliance in multiple directions, or the electrodes may
comprise carbon fibrils, carbon nanotubes, mixtures of ionically
conductive materials or colloidal suspensions. Colloidal
suspensions contain submicrometer sized particles, such as
graphite, silver and gold, in a liquid vehicle.
The polymer may be a commercially available product such as a
commercially available acrylic elastomer film. It may be a film
produced by casting, dipping, spin coating or spraying.
Textured electrodes known in the prior art may, alternatively, be
patterned photolithographically. In this case, a photoresist is
deposited on a pre-strained polymer and patterned using a mask.
Plasma etching may remove portions of the electroactive polymer not
protected by the mask in a desired pattern. The mask may be
subsequently removed by a suitable wet etch. The active surfaces of
the polymer may then be covered with the thin layer of gold
deposited by sputtering, for example.
Producing electroactive polymers, and in particular rolled
actuators, using the technique described in U.S. Pat. No. 6,376,971
and U.S. Pat. No. 6,891,317 has the disadvantage that direction of
compliance of the corrugated electrodes is very difficult to
control.
Finally, in order to obtain the necessary compliance using the
prior art technology, it is necessary to use materials having a
relatively high electrical resistance for the electrodes. Since a
rolled actuator with a large number of windings will implicitly
have very long electrodes, the total electrical resistance for the
electrodes will be very high. The response time for an actuator of
this kind is given by .tau.=RC, where R is the total electrical
resistance of the electrodes and C is the capacitance of the
composite. Thus, a high total electrical resistance results in a
very long response time for the actuator. Thus, in order to obtain
an acceptable response time, the number of windings must be
limited, and thereby the actuation force is also limited, i.e.
response time and actuation force must be balanced when the
actuator is designed.
SUMMARY OF THE INVENTION
It is an object of a preferred embodiment of the invention to
provide a composite for a transducer, which composite facilitates
an increased actuation force as compared to similar prior art
composites for transducers. It is a further object of the invention
to provide a composite which facilitates increased compliance of
the transducer in one direction, facilitates an improved reaction
time as compared to similar prior art composites for transducers,
and which potentially provides an increased lifetime of the
transducer in which it is applied. In this regards, compliance
means that it is easy to stretch the composite in one
direction.
According to a first aspect of the invention the above and other
objects are fulfilled by a composite in which a film of a
dielectric material has a first surface and second surface, at
least the first surface comprising a surface pattern of raised and
depressed surface portions. A first electrically conductive layer
is deposited onto the surface pattern and forms an electrode layer
thereon. To enable elongation of the composite in one well defined
direction, i.e. to provide compliance, the electrically conductive
layer has a corrugated shape which renders the length of the
electrically conductive layer in a lengthwise direction, longer
than the length of the composite as such in the lengthwise
direction. The corrugated shape of the electrically conductive
layer thereby facilitates that the composite can be stretched in
the lengthwise direction without having to stretch the electrically
conductive layer in that direction, but merely by evening out the
corrugated shape of the electrically conductive layer. According to
the invention, the corrugated shape of the electrically conductive
layer is a replica of the surface pattern of the film.
Since the conductive layer is deposited onto the surface pattern of
the film and is formed by the shape thereof, a very precise shape
of the corrugation of the conductive layer can be defined, and an
improved compliance towards deformation in a specific direction can
be provided by a suitable design of the surface pattern on the
film. Accordingly, the composite can facilitate increased actuation
forces, or in general an increased rate of conversion between
mechanical and electrical energies, increased lifetime and improved
reaction time when the composite is used in a transducer.
In the prior art composites, the pattern of the film and electrode
is provided by stretching the film prior to the application of the
electrode on the surface of the film. When the stretch of the film
is released, the electrode wrinkles, and since the electrode is
bonded to the film, the surface of the film wrinkles with the
electrode. Since the shape of the electrically conductive layer in
accordance with the present invention is a replica of the shape of
the surface pattern of the film, it may be provided that the shape
of the composite as such is unaffected by the contact and bonding
between the electrically conductive layer and the film. It may
further be provided that the shape is essentially unaffected by
elastic moduli of the electrically conductive layer and film. It
may further be unaffected by the thickness of the electrically
conductive layer and film. This provides a larger degree of freedom
with respect to the selection of materials for the film and for the
electrically conductive layer and thus enables improved performance
of the composite when used in a transducer.
To restrict deformation of the composite in other directions than
the direction of compliance, the electrically conductive layer may
have a modulus of elasticity much higher than a modulus of
elasticity of the film. Accordingly, the electrically conductive
layer resists elongation and thus prevents deformation of the
composite in directions in which the length of the electrically
conductive layer corresponds to the length of the composite as
such.
The composite is provided with at least one surface with an
electrically conductive layer. One surface is not sufficient to
create an active composite in which the film can be deformed by a
potential difference between two electrically conductive layers on
opposite surfaces. In the following, the abbreviation, inactive
composite is used for a composite with a single surface
electrically conductive layer and an active composite is used for a
composite with a double surface electrically conductive layer.
The composite according to the invention can be made active by
applying additional composites in a layered structure. A first
electrically conductive layer of one layer becomes a second
electrically conductive layer of an adjacent layer. This is
described in further details later. Another way of making the
composite active is to apply an additional electrically conductive
layer onto an opposite surface of the film.
In order to benefit the most from the composite, e.g. in a
transducer comprising a composite which is curled or wound to form
a rolled structure with a larger number of layers or windings, it
is preferred to provide the composite as a very long web. In this
context, a web denotes something which is potentially unlimited in
length and which can therefore be provided as a spooled product
similar to cling-wrap, cling-film or household foil. In general,
the web is at least 10 times longer in a lengthwise direction than
in a perpendicular crosswise direction, but it may even be 100,
1000 or more times longer in the lengthwise direction.
The surface pattern and thus the corrugations define a compliance
direction in which the composite is mostly compliant, and this
direction could be anything from the lengthwise direction to the
crosswise direction.
As mentioned above, the surface pattern leads to a designed
anisotropic compliance. The anisotropic compliance is caused by the
electrically conductive layer. Since the surface pattern creates an
electrically conductive layer with a length in one direction being
substantially longer than the length of the composite as such, the
composite can be stretched in one direction without stretching the
electrically conductive layer. This provides the compliance in this
direction. Conversely, the lack of compliance in other directions
is also provided by the same electrically conductive layer because
it is substantially less elastically deformable than the film.
Preferably, the ratio between a modulus of elasticity of the
electrically conductive layer and a modulus of elasticity of the
film is larger than 200.
The production of the film can be made in a moulding or coating
process after which the electrically conductive layer is applied so
that it follows the surface pattern of the film. The electrically
conductive film is formed as a replica of the film's surface
pattern when the electrically conductive layer is deposited onto
the film. The film, on the other hand, could likewise be made by
providing a liquid material onto a shape defining element, e.g. in
a moulding, coating or painting process, and subsequently allowing
the liquid material to cure to form a film. In the present context
the term `dielectric material` should be interpreted to mean a
material having a relative permittivity, .di-elect cons..sub.r,
which is larger than or equal to 2.
The dielectric material may be a polymer, e.g. an elastomer, such
as a silicone elastomer, such as a weak adhesive silicone. A
suitable elastomer is Elastosil RT 625, manufactured by
Wacker-Chemie. Alternatively, Elastosil RT 622 or Elastosil RT 601,
also manufactured by Wacker-Chemie may be used. As an alternative,
other kinds of polymers may be chosen.
In the case that a dielectric material which is not an elastomer is
used, it should be noted that the dielectric material should have
elastomer-like properties, e.g. in terms of elasticity. Thus, the
dielectric material should be deformable to such an extent that the
composite is capable of deflecting and thereby pushing and/or
pulling due to deformations of the dielectric material.
The film and the electrically conductive layer may have a
relatively uniform thickness, e.g. with a largest thickness which
is less than 110 percent of an average thickness of the film, and a
smallest thickness which is at least 90 percent of an average
thickness of the film. Correspondingly the first electrically
conductive layer may have a largest thickness which is less than
110 percent of an average thickness of the first electrically
conductive layer, and a smallest thickness which is at least 90
percent of an average thickness of the first electrically
conductive layer. In absolute terms, the electrically conductive
layer may have a thickness in the range of 0.01 .mu.m to 0.1 .mu.m,
such as in the range of 0.02 .mu.m to 0.09 .mu.m, such as in the
range of 0.05 .mu.m to 0.07 .mu.m. Thus, the electrically
conductive layer is preferably applied to the film in a very thin
layer. This facilitates good performance and facilitates that the
electrically conductive layer can follow the corrugated pattern of
the surface of the film upon deflection.
The electrically conductive layer may have a thickness in the range
of 0.01-0.1 .mu.m, and the film may have a thickness between 10
.mu.m and 200 .mu.m, such as between 20 .mu.m and 150 .mu.m, such
as between 30 .mu.m and 100 .mu.m, such as between 40 .mu.m and 80
.mu.m. In this context, the thickness of the film is defined as the
shortest distance from a point on one surface of the film to an
intermediate point located halfway between a crest and a trough on
a corrugated surface of the film.
The electrically conductive layer may have a resistivity which is
less than 10.sup.-4.OMEGA.cm. By providing an electrically
conductive layer having a very low resistivity the total resistance
of the electrically conductive layer will not become excessive,
even if a very long electrically conductive layer is used. Thereby,
the response time for conversion between mechanical and electrical
energy can be maintained at an acceptable level while allowing a
large surface area of the composite, and thereby obtaining a large
actuation force when the composite is used in an actuator. In the
prior art, it has not been possible to provide corrugated
electrically conductive layers with sufficiently low electrical
resistance, mainly because it was necessary to select the material
for the prior art electrically conductive layer with due
consideration to other properties of the material in order to
provide the compliance. By the present invention it is therefore
made possible to provide compliant electrically conductive layers
from a material with a very low resistivity. This allows a large
actuation force to be obtained while an acceptable response time of
the transducer is maintained.
The electrically conductive layer may preferably be made from a
metal or an electrically conductive alloy, e.g. from a metal
selected from a group consisting of silver, gold and nickel.
Alternatively other suitable metals or electrically conductive
alloys may be chosen. Since metals and electrically conductive
alloys normally have a very low resistivity, the advantages
mentioned above are obtained by making the electrically conductive
layer from a metal or an electrically conductive alloy.
The dielectric material may have a resistivity which is larger than
10.sup.10.OMEGA.cm. Preferably, the resistivity of the dielectric
material is much higher than the resistivity of the electrically
conductive layer, preferably at least 10.sup.14-10.sup.18 times
higher.
The corrugated pattern may comprise waves forming crests and
troughs extending in one common direction, the waves defining an
anisotropic characteristic facilitating movement in a direction
which is perpendicular to the common direction. According to this
embodiment, the crests and troughs resemble standing waves with
essentially parallel wave fronts. However, the waves are not
necessarily sinusoidal, but could have any suitable shape as long
as crests and troughs are defined. According to this embodiment a
crest (or a trough) will define substantially linear contour-lines,
i.e. lines along a portion of the corrugation with equal height
relative to the composite in general. This at least substantially
linear line will be at least substantially parallel to similar
contour lines formed by other crest and troughs, and the directions
of the at least substantially linear lines define the common
direction. The common direction defined in this manner has the
consequence that anisotropy occurs, and that movement of the
composite in a direction perpendicular to the common direction is
facilitated, i.e. the composite, or at least an electrically
conductive layer arranged on the corrugated surface, is compliant
in a direction perpendicular to the common direction. In connection
with the potentially unlimited web, the wave crests and troughs may
extend e.g. in the lengthwise direction or in the crosswise
direction.
Preferably, the compliance of the composite in the compliant
direction is at least 50 times larger than its compliance in the
common direction, i.e. perpendicularly to the compliant
direction.
The waves may have a shape which is periodically repeated. In one
embodiment, this could mean that each of the crests and each of the
troughs are at least substantially identical. Alternatively, the
periodicity may be obtained on a larger scale, i.e. the repeated
pattern may be several `wavelengths` long. For instance, the
wavelength, the amplitude the shape of the crests/troughs, etc. may
be periodically repeated. As an alternative, the shape of the waves
may be non-periodical.
Each wave may define a height being a shortest distance between a
crest and neighbouring troughs. In this case each wave may define a
largest wave having a height of at most 110 percent of an average
wave height, and/or each wave may define a smallest wave having a
height of at least 90 percent of an average wave height. According
to this embodiment, variations in the height of the waves are very
small, i.e. a very uniform pattern is obtained.
According to one embodiment, an average wave height of the waves
may be between 1/3 .mu.m and 20 .mu.m, such as between 1 .mu.m and
15 .mu.m, such as between 2 .mu.m and 10 .mu.m, such as between 4
.mu.m and 8 .mu.m.
Alternatively or additionally, the waves may have a wavelength
defined as the shortest distance between two crests, and the ratio
between an average height of the waves and an average wavelength
may be between 1/30 and 2, such as between 1/20 and 3/2, such as
between 1/10 and 1.
The waves may have an average wavelength in the range of 1 .mu.m to
20 .mu.m, such as in the range of 2 .mu.m to 15 .mu.m, such as in
the range of 5 .mu.m to 10 .mu.m.
A ratio between an average height of the waves and an average
thickness of the film may be between 1/50 and 1/2, such as between
1/40 and 1/3, such as between 1/30 and 1/4, such as between 1/20
and 1/5.
A ratio between an average thickness of the electrically conductive
layers and an average height of the waves may be between 1/1000 and
1/50, such as between 1/800 and 1/100, such as between 1/700 and
1/200.
In a preferred embodiment of the invention the composite is
designed by optimising the parameters defined above in such a
manner that dielectric and mechanical properties of the film as
well as of the electrically conductive layer material are taken
into consideration, and in such a manner that a composite having
desired properties is obtained. Thus, the average thickness of the
film may be selected with due consideration to the relative
permittivity and breakdown field of the film on the one hand, and
electrical potential difference between the electrically conductive
layers on the other hand. Similarly, the height of the crests may
be optimised with respect to the thickness of the film in order to
obtain a relatively uniform electric field distribution across a
film of dielectric material arranged between the electrically
conductive layers. Furthermore, electrically conductive layer
thickness, average wavelength, and wave height may be optimised in
order to obtain a desired compliance. This will be described
further below with reference to the drawings.
As previously mentioned, the composite could become electroactive
by the provision of an additional, second, electrically conductive
layer arranged opposite to the first electrically conductive layer
relative to the film. The second electrically conductive layer may,
like the first layer, have a corrugated shape which could be
provided as a replica of a surface pattern of the film.
Alternatively, the second electrically conductive layer is
substantially flat. If the second electrically conductive layer is
flat, the composite will only have compliance on one of its two
surfaces while the second electrically conductive layer tends to
prevent elongation of the other surface. This provides a composite
which bends when an electrical potential is applied across the two
electrically conductive layers.
Another way of making the composite electroactive is by combining
several composites into a multilayer composite with a laminated
structure. In a second aspect, the invention provides a multilayer
composite comprising at least two layers of composite, each
composite layer comprising:
a film made of a dielectric material and having a front surface and
rear surface, the front surface comprising a surface pattern of
raised and depressed surface portions, and
a first electrically conductive layer being deposited onto the
surface pattern, the electrically conductive layer having a
corrugated shape which is formed by the surface pattern of the
film.
In this structure, an electrode group structure may be defined,
such that every second electrically conductive layer becomes an
electrode of a first group and every each intermediate electrically
conductive layer becomes an electrode of a second group of
electrodes. A potential difference between the electrodes of the
two groups will cause a deformation of the film layers located
there between, and the composite is therefore electroactive. In
such a layered configuration, a last layer will remain inactive.
Accordingly, a multilayer composite with three layers comprises 2
active layers, a multilayer composite with 10 layers comprises 9
active layers, etc.
If the electrically conductive layers are deposited on front
surfaces of the films, it may be an advantage to arrange the layers
with the rear surfaces towards each other. In this way, the
multilayer composite becomes less vulnerable to faults in the film.
If the film in one layer has a defect which enables short
circuiting of electrodes on opposite surfaces thereof, it would be
very unlikely if the layer which is arranged with its rear surface
against the film in question has a defect at the same location. In
other words, at least one of the two films provides electrical
separation of the two electrically conductive layers.
The multilayer composite can be made by arranging the composite
layers in a stack and by applying an electrical potential
difference between each adjacent electrically conductive layer in
the stack so that the layers are biased towards each other while
they are simultaneously flattened out. Due to the physical or
characteristic properties of the film, the above method may bond
the layers together. As an alternative or in addition, the layers
may be bonded by an adhesive arranged between each layer. The
adhesive should preferably be selected not to dampen the compliance
of the multilayer structure. Accordingly, it may be preferred to
select the same material for the film and adhesive, or at least to
select an adhesive with a modulus of elasticity being less than the
modulus of elasticity of the film.
The composite layers in the multilayer composite should preferably
be identical to ensure a homogeneous deformation of the multilayer
composite throughout all layers, when an electrical field is
applied. Furthermore, it may be an advantage to provide the
corrugated pattern of each layer either in such a way that wave
crests of one layer are adjacent to wave crests of the adjacent
layer or in such a way that wave crests of one layer are adjacent
to troughs of the adjacent layer.
In a third aspect, the invention provides a method of making a
composite, the method comprising:
providing a shape defining element having a surface pattern of
raised and depressed surface portions,
providing a liquid polymer composition onto the surface pattern
curing the liquid polymer to form a polymeric film having a surface
with a replicated pattern of raised and depressed surface portions,
and
depositing a first electrically conductive layer onto the
replicated surface pattern so that the electrically conductive
layer is shaped by the replicated pattern.
Since the shape of the electrically conductive layer is obtained
from the pattern of the film, and the pattern of the film is
obtained from a shape defining element e.g. by a moulding, coating,
painting or by any similar process of shape replication, the shape
of the electrically conductive layer can be designed specifically
for a certain purpose. Accordingly, the composite may provide
improved performance if used in a transducer.
The film could be made from a liquid dielectric material, e.g. a
liquid polymer in a reverse roll process, a gravure roll process or
a slot die coating process. The liquid polymer could be thinned
with a solvent to facilitate films of very low thickness and to
cross-link the polymer, the film could be exposed to heat or
ultraviolet light.
To improve adhesion of the electrically conductive layer, the film
may be treated with plasma. The treatment could be conducted with a
glow discharge which is known to generate mild plasma. To this end
argon plasma is preferred. Prior to the deposition of the
electrically conductive layer on the film, an adhesion promoter
could be applied to the film. The adhesion promoter is applied
after the plasma treatment of the film. Examples of such promoters
are a layer of chromium or a layer of titanium. The adhesion
promoter could be applied to the film e.g. in a physical vapour
deposition process.
Plasma cleaning is a critical step in the metallization process of
elastomer films. It enhances adhesion of the deposited material.
However, not any plasma is appropriate for treating the elastomer
film, and the plasma should therefore be selected carefully. As
mentioned above, argon plasma is preferred. Plasma treatments are
known to form thin and very stiff silicate "glassy" layers at the
elastomer interface. When an electrically conductive layer is
subsequently applied, the result is corrugated electrodes with
limited compliance and composites which cannot be stretched very
much because of the risk of cracking the stiff electrodes. We have
chosen the argon plasma treatment which is not reactive because
argon is a noble gas. However, residues of oxygen and other
reactive gases in the vacuum deposition chamber combined with the
argon plasma, may be responsible for a little reactivity. We
optimise the pressure of argon in the vacuum chamber and the
parameters of the mild glow discharge, as well as the duration of
the treatment in such a way that the deposited metal coating
adheres very well to the elastomer film. The resulting corrugated
electrode is very compliant and the composite can be stretched
without damaging the electrode.
As mentioned above, the electrically conductive layer is very thin
and the thickness of the electrically conductive layer is very
small as compared to the thickness of the film. The electrically
conductive layer could be deposited on the film in a physical
vapour deposition process, e.g. a sputtering process or an electron
beam process. Alternatively, a spray coating process may be
applied. To obtain a precise thickness, the thickness is controlled
by quartz crystal micro balance.
Quartz crystal micro balance is a thickness measurement technique
that is commonly used in physical vapour deposition. It allows for
controlling the thickness of the deposited coating, e.g. a metal
coating or similar, with accuracies in the sub-nanometer range.
In a fourth aspect, the invention provides a transducer made at
least partly from a composite or from a multilayer composite of the
above-mentioned kind.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in further detail with
reference to the accompanying drawings in which:
FIGS. 1a and 1b illustrate continuous rolls of spooled composites
according to embodiments of the invention,
FIG. 1c is a perspective view of a portion of a composite according
to an embodiment of the invention,
FIGS. 2a-2f are cross sectional views of a portion of composites
according to embodiments of the invention,
FIG. 2g is an enlarged section of FIG. 2a/2b/2c/2d/2e/2f,
FIGS. 3a and 3b show an electroactive composite being exposed to
zero electrical potential difference and being exposed to a high
electrical potential difference,
FIGS. 4a-4c illustrate the effect of exposing the electroactive
composite of FIG. 3a to a high electrical potential difference as
shown in FIG. 3b,
FIGS. 5a and 5b illustrate an example of lamination of composites
according to an embodiment of the invention, thereby forming an
electroactive multilayer composite,
FIGS. 5c and 5d illustrate an electroactive multilayer composite
being exposed to zero electrical potential difference and being
exposed to a high electrical potential difference,
FIGS. 6a and 6b illustrate another example of lamination of
composites according to an embodiment of the invention, thereby
forming an electroactive multilayer composite,
FIGS. 6c and 6d illustrate another electroactive multilayer
composite being exposed to zero electrical potential difference and
being exposed to a high electrical potential difference,
FIGS. 7-9 illustrate examples of lamination principles of
composites according to embodiments of the invention,
FIGS. 10a and 10b illustrate examples of rolled electroactive
composites,
FIG. 11a illustrates an example of a portion of a composite
according to an embodiment of the invention, the composite being
particularly suitable for a composite having a rolled
structure,
FIG. 11b illustrates an example of a portion of a composite
according to an embodiment of the invention, the composite being
particularly suitable for a composite having a folded
structure,
FIGS. 12a-12c illustrate a process of making the composite of FIG.
11 and some of the tools needed for the production,
FIG. 13a illustrates the composite of FIG. 11a formed as a rolled
composite,
FIG. 13b illustrates the composite of FIG. 11b formed as a folded
composite,
FIGS. 14a and 14b illustrate lamination of the composite shown FIG.
11 by folding of the composite,
FIGS. 15a-15c are perspective views of direct axially actuating
transducers according to embodiments of the invention,
FIGS. 16a-16c are graphs illustrating force as a function of stroke
in a direct actuating transducer according to an embodiment of the
invention,
FIGS. 17a and 17b are perspective views of direct radially
actuating transducers according to embodiments of the
invention,
FIG. 18a illustrates lamination of a composite to form a flat
tubular structure,
FIG. 18b illustrate the flat tubular structure of FIG. 18a being
pre-strained,
FIGS. 19a-19c are perspective views of an actuating transducer
having a flat structure,
FIGS. 20a-20e illustrate actuating transducers provided with a
preload,
FIGS. 21a and 21b illustrate two actuating transducers having a
flat tubular structure, the transducers being provided with
mechanical connection,
FIG. 22 illustrates the principle of space-shifted laminated layers
of composites,
FIG. 23 illustrates laminated electroactive multilayer composites
provided with electrical contact portions and electrical
connectors,
FIGS. 24 and 25 illustrate two examples of electroactive multilayer
composites provided with electrical contact portions,
FIGS. 26-29 illustrate examples of transducers provided with
electrical contact portions,
FIG. 30 illustrates different electrical connectors,
FIGS. 31-35 illustrate electroactive composites provided with
contact electrodes, and
FIGS. 36a-36c is a process diagram describing a manufacturing
process of a transducer according to an embodiment of the
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b illustrate continuous rolls of spooled composites 1
according to embodiments of the invention, and FIG. 1c is a
perspective view of a portion of a composite 1. The proportions of
the composite are distorted in order to illustrate different
elements of the composite 1. The composite 1 comprises a film 2
made of a dielectric material having a surface 3 provided with a
pattern of raised and depressed surface portions, thereby forming a
designed corrugated profile of the surface 3. An electrically
conductive layer 4 has been applied to the surface 3, the
electrically conductive material being deposited so that the
electrically conductive layer is formed according to the pattern of
raised and depressed surface portions. In terms of everyday
physical things, the film 2 resembles in some aspects household
wrapping film. It has a similar thickness and is comparably pliable
and soft. However, it is more elastic than such a film, and has a
marked mechanical anisotropy as will be explained in the
following.
The dielectric material may be an elastomer or another material
having similar characteristics.
Due to the pattern of raised and depressed surface portions, the
electrically conductive layer 4 may even out as the film 2 expands,
and recover its original shape as the film 2 contracts along the
direction defined by arrows 5 without causing damage to the
electrically conductive layer 4, this direction thereby defining a
direction of compliance. Accordingly, the composite 1 is adapted to
form part of a compliant structure capable of withstanding large
strains.
As described above, the corrugated surface profile is directly
impressed or moulded into the dielectric film 2 before the
electrically conductive layer is deposited. The corrugation allows
the manufacturing of a compliant composite using electrode
materials of high elastic modulii, e.g. metal electrode. This can
be obtained without having to apply pre-stretch or pre-strain to
the dielectric film 2 while applying the electrically conductive
layer 4, and the corrugated profile of the finished composite 1
does not depend on strain in the dielectric film 2, nor on the
elasticity or other characteristics of the electrically conductive
layer 4. Accordingly, the corrugation profile is replicated over
substantially the entire surface 3 of the dielectric film 2 in a
consistent manner, and it is possible to control this replication.
Furthermore, this approach provides the possibility of using
standard replication and reel-to-reel coating, thereby making the
process suitable for large-scale production. For instance, the
electrically conductive layer 4 may be applied to the surface 3 of
the dielectric film 2 using standard commercial physical vapour
deposition (PVD) techniques. An advantage of this approach is that
the anisotropy is determined by design, and that the actual
anisotropy is obtained as a consequence of characteristics of the
corrugated profile which is provided on the surface 3 of the
dielectric film 2 and the electrically conductive layer 4 which
follows the corrugated profile.
The composite 1 shown in FIG. 1c is designed to have a compliance
in the range of the compliance of the dielectric film 2 in the
direction defined by arrows 5, and a stiffness in the range of the
stiffness of the electrically conductive layer 4 in a direction
defined by arrows 6. In FIG. 1a, the compliance direction is along
the length of the composite 1, whereas the compliance direction of
FIG. 1b is across the composite 1. This is indicated by the thin
lines across the composite 1 in FIG. 1a and along the composite 1
in FIG. 1b, which thin lines represents the pattern of raised and
depressed surface portions forming the corrugated profile. The
composite 1 may be produced in very long lengths, so called
"endless" composites which may be stored as spools as shown in
FIGS. 1a and 1b. Such semi finished goods may be used for the
production of transducers and the like, e.g. actuators.
FIGS. 2a-2f illustrate a portion of a sectional view of composites
1 according to embodiments of the invention, with hatchings omitted
for the sake of clarity. As indicated by the symmetry line 10 at
the bottom of each portion, each portion only shows half a
composite 1. Furthermore, an electrically conductive layer 4 may be
deposited on the lower surface of the dielectric film 2, which
lower surface may also define a corrugated surface, thereby forming
an electroactive composite, i.e. at least two electrically
conductive layers being separated by a dielectric film.
Furthermore, each portion only shows a small portion lengthwise of
each composite. For illustration purposes the proportions of FIGS.
2a-2g are out of order. FIG. 2g illustrates an enlarged section of
FIG. 2a/2b/2c/2d/2e/2f. The composite 1 shown in FIGS. 2a-2g could,
e.g., be the composite 1 of FIG. 1a. Thus, the composite 1
comprises a dielectric film 2 made of a dielectric material having
a surface 3 provided with a pattern of raised and depressed surface
portions, thereby forming a corrugated profile of the surface 3.
The surface 3 is provided with an electrically conductive layer
(shown in FIG. 2g) forming a directionally compliant composite as
described above. As shown in FIGS. 2a-2f, the pattern of raised and
depressed surface portions may be designed having various
shapes.
The corrugated profile may be represented by a series of well
defined and periodical sinusoidal-like three dimensional
microstructures. Alternatively, the corrugated profile may have a
triangular or a square profile. The mechanical compliance factor,
Q, of the corrugated electrode is determined by the scaling ratio
between the depth d of the corrugation and the thickness h (see
FIG. 2g) of the electrically conductive layer 4, and by the scaling
ratio between the depth d of the corrugation and its period P. The
most dominating factor is the scaling ratio between the height d of
the corrugation and the thickness h of the electrically conductive
layer 4. The larger the compliance factor, the more compliant the
structure is. It has been found by the inventors of the present
invention, that if perfect compliance is assumed, for a scaling
ratio between the depth d of the corrugation and its period P, a
sinus profile could theoretically elongate approximately 32%, a
triangular profile approximately 28% and a square profile
approximately 80% compared to the original length. However, in
reality this will not be the case since the square profile
comprises vertical and horizontal beams, which will result in
different compliances, because the vertical beams will bend and
thereby generate a very compliant movement in the displacement
direction, while the horizontal beams will be much stiffer, since
they extent in the displacement direction. It is therefore often
desirable to choose the sinus profile.
In the composite 1 shown in FIGS. 2a-2f, the corrugated pattern
impressed or moulded into the dielectric film 2 can be represented
by a series of well defined and periodical sinusoidal-like three
dimensional microstructures. The corrugation profile is formed at
the upper surface 3 of the film 2 as shown in FIGS. 2a-2f. As
indicated by the symmetry line 10, a second corrugation profile is
formed at the lower surface (not shown) of the film. In FIGS.
2a-2f, the section runs along the direction of compliance.
Perpendicularly to the direction of compliance parallel straight
lines represent tops and bottoms of the raised and depressed
surface portions, i.e. wave crests or troughs of the
sinusoidal-like microstructure. This appears more clearly from
FIGS. 1a and 1c. Along these parallel straight lines, the
compliancy is very low, i.e. for all practical purposes the
composite 1 is not compliant in this direction. In other words,
this design represents a one dimensional corrugation which, upon
application of the electrically conductive layers, transforms the
dielectric film 2 into an electrocative composite 1 with
anisotropic compliance, wherein the film is free to contract or
elongate, while a perpendicularly arranged cross-plane direction is
`frozen` due to the built-in boundary conditions given by the
mechanical resistance of the electrically conductive layers 4.
In FIGS. 2a-2g, d denotes an average or representative corrugation
depth, i.e. an average or representative distance between a raised
portion and a neighbouring depressed portion of the pattern. H
denotes an average thickness of the dielectric film 2, and h
denotes an average thickness of the electrically conductive layer
4. In a preferred embodiment, the average thickness H of the
dielectric film 2 is in the range of 10 .mu.m-100 .mu.m. FIGS.
2a-2c show composites 1 having different corrugation depth d,
whereas the corrugation period P is substantially identical for the
three composites shown. Comparing the composites 1 of FIGS. 2d and
2e, the corrugation depth d is substantially identical, whereas the
corrugation period P of the composite 1 in FIG. 2e is larger than
the corrugation period P of the composite 1 shown in FIG. 2d.
Compared hereto, the composite 1 of FIG. 2f has a smaller
corrugation depth d and a larger corrugation period P.
The properties of the dielectric films 2 with anisotropic
corrugated compliant metallic electrodes in the form of
electrically conductive layers 4 as described in accordance with
the present invention are optimised by design according to design
rules developed by the inventors. These design rules take into
consideration the dielectric and mechanical properties of the
dielectric material and of the material of the electrically
conductive layer.
The relative permittivity and breakdown field of the dielectric
material on the one hand and electrical potential difference
between electrodes on the other hand are the design parameters that
determine the range of the average thickness, H of the dielectric
film 2. The characteristic properties of the dielectric material
are typically supplied by dielectric material manufacturers like
Wacker-Chemie and Dow Corning.
Corrugation depth, d, is optimised with respect to the dielectric
film thickness, H, in order to obtain a relatively uniform electric
field distribution across the dielectric film situated between the
electrodes. Such optimisation step is done using finite element
simulations. A high d/H ratio corresponds to a non uniform electric
field distribution and a low d/H ratio corresponds to a relatively
uniform electric field distribution.
Anisotropy and compliance properties are the combined result of the
shape and topology given to the surface of the dielectric film,
e.g. an elastomer film, by a moulding process on one hand and the
electrically conductive layer that takes up the corrugation shape
on the other hand. Electrode layer thickness, h, and corrugation
period, P, are optimised with respect to the corrugation depth, d,
in order to obtain a dielectric film with metallic electrodes that
is compliant in one `in the plane` direction and almost not
compliant in the transverse `in the plane` direction. A film that
is very compliant in one direction is a film that can be stretched
or elongated very much in this direction by applying a relatively
low level of forces in this direction without the risk of damaging
the electrodes, and a film that will have very limited elongation
in the transverse direction when a force is applied in this
transverse direction. In order to optimise electrode compliance,
the d/P and h/d ratios have to be optimized. High d/P ratios result
in very compliant electrodes and low d/P ratios result in less
compliant electrodes. High h/d ratios result in less compliant
electrodes and low h/d ratios result in very compliant electrodes.
The degree of anisotropy of the dielectric film with corrugated
electrodes is determined by the compliance ratio between the
direction in which the composite is compliant and the transverse
direction in which the composite is almost not compliant. High
compliance ratios result in very anisotropic structures and low
ratios result in isotropic-like structures.
Once the ranges for the design parameters (H, d, h and P) are
specified according to the above description, it is possible to
predict the performance of the dielectric film with metallic
electrodes in the form of electrically conductive layers in terms
of how compliant and what maximum elongation in the compliant
direction it can undergo and what the actuation forces will be.
Stiffness in the transverse direction can be predicted as well. A
refinement process for these parameters can be done if
necessary.
It should be noted that for a given actuation force, actuators
manufactured in accordance with the present invention, i.e. made
from a dielectric material with electrodes deposited thereon, has a
much lower weight, i.e. at least a factor five smaller, than
conventional actuators, such as magnetic actuators, capable of
providing a comparable actuation force. This is very important for
applications where actuator volume and weight are of relevance.
Once all design parameters are optimised, a mould is designed
according to the exact specifications for the corrugation
topology.
Based on finite element electrostatic simulations, the inventors of
the present invention have found that the ratio d/H should be in
the range of 1/30-1/2. For example, having a ratio of 1/5 and a
corrugation depth of approximately 4 .mu.m, the thickness of the
dielectric film 2 will be approximately 20 .mu.m. Furthermore, the
ratio between the corrugation depth d and the period P of the
corrugations, d/P, and the ratio between the thickness h of the
electrically conductive layer and the corrugation depth d, h/d, are
important ratios directly affecting the compliance of the
electrode. In preferred embodiments, the ratio d/P is in the range
of 1/50-2, whereas the ratio h/d is in the range of
1/1000-1/50.
Another issue to take into consideration when defining the average
thickness H of the dielectric film 2 is the so-called breakdown
electric field related to dielectric materials. When an
electrically conductive layer 4 is deposited on each surface of the
dielectric film 2 thereby forming an electroactive composite, there
is a maximum value for the voltage, V between these electrically
conductive layers, for a given material thickness, H, i.e. a
distance corresponding to the thickness, H, of the dielectric film
2, in order not to exceed the breakdown electric field, V/H, of the
material. When the dielectric film 2 presents large variations in
thickness across a surface area 3, then, for a given voltage
between the electrically conductive layers, electric field and
thickness variations will be of the same order of magnitude. As a
consequence, parts of the dielectric film 2 having a higher local
electric field will elongate more than those with a smaller local
electric field. Furthermore, in situations where a transducer in
which the composite 1 is operated close to a breakdown field, such
variations may be damaging to the transducer, because parts of the
dielectric film 2 will be subjected to electric fields which are
larger than the breakdown field. Accordingly, it is very important
to reduce the average thickness variations to the greatest possible
extent when processing the dielectric film 2. For processing
reasons a 10% average thickness variation is considered acceptable.
When processing transducers with corrugated electrodes by design,
i.e. in accordance with the present invention, these values can be
controlled in a relatively accurate manner.
FIGS. 3a and 3b illustrate an electroactive composite 1 comprising
two electrically conductive layers 4 separated by a dielectric film
2 being exposed to zero electrical potential difference (FIG. 3a)
and being exposed to a high electrical potential difference (FIG.
3b). As illustrated in FIG. 3b the dielectric film 2 is expanded,
while the electrically conductive layers 4 are evened out, when
exposed to an electrical potential difference. This is shown in
detail in FIGS. 4a-4c which illustrate portions of a section of the
electroactive composite 1 at different steps in time, with
hatchings omitted for the sake of clarity. A line of symmetry 10 is
indicated at the bottom of each figure, illustrating that the
composite 1 is an electroactive composite having an electrically
conductive layer 4 deposited on each surface. FIG. 4a illustrate
the electroactive composite 1 being exposed to zero electrical
potential difference, the corrugation depth being the designed
depth d and the corrugation period being the designed period P. In
FIG. 4b it is illustrated that the dielectric film 2 is expanded in
the compliance direction resulting in a reduced thickness H' of the
film. Furthermore, the electrically conductive layer 4 is evened
out resulting in a smaller corrugation depth d' and a larger
corrugation period P'. FIG. 4c illustrate the electroactive
composite 1 at a later time step, the thickness H'' of the film 2
being even more reduced, the corrugation depth d'' being even
smaller and the corrugation period P'' being larger.
It should be noted that capacitors produced in accordance with the
present invention exhibit a `self-healing` mechanism. A
self-healing mechanism is characteristic of capacitors with very
thin electrodes. It occurs when the dielectric material of the
capacitor presents defects such as inclusions, pinholes, etc. For
such a capacitor with a given thickness, when the applied potential
difference between electrodes approaches the so-called breakdown
voltage defined above, the average electric field approaches the
critical breakdown field. However, in regions with defects, it will
indeed exceed this critical breakdown field, and a cascading effect
due to accelerated and colliding charges across dielectric film
thickness at the positions of the defects occurs, thereby inducing
a high in-rush transient current across the dielectric material.
This results in a local transient over-heating with characteristic
times in the microseconds range or much below, which is enough to
"deplete/evaporate" the material of the very thin opposite
electrodes at the positions of the defects and their close
vicinity. This results in areas around defects where there is no
more electrode material. Moreover the dimension of the areas with
depleted electrode material increases with the local field.
However, the capacitor as such is not damaged and continues to
operate. Thus, the reference to `self-healing`. As long as the
depleted areas represent in total a very negligible fraction of the
entire area of the capacitor, this will have very little
consequence on the performance of the capacitor. Self-healing does
not take place if the capacitor is made with thick electrodes,
because the level of local over-heating is not sufficient to
deplete the thick electrode material at the defects. In that case,
when the critical breakdown is reached, consequent and instant
damage of the capacitor occurs. In practice, the inventors of the
present invention have made metallic electrodes with thickness up
to 0.2 .mu.m and always observed self-healing, even when operating
the capacitor above breakdown. This does not cause any substantial
damage to the capacitor, and the capacitor therefore continues to
operate.
FIGS. 5-9 illustrate examples of lamination of composites 1 thereby
creating multilayer composites. As shown in FIGS. 5a and 6a, an
electroactive multilayer composite 15, 16 comprises at least two
composites 1, each composite 1 comprising a dielectric film 2
having a front surface 20 and a rear surface 21, the rear surface
21 being opposite to the front surface 20. The front surface 20
comprises a surface pattern 3 of raised and depressed portions and
a first electrically conductive layer (not shown) covering at least
a portion of the surface portion 3. FIGS. 5a and 6a only show a
portion of a multilayer composite 15 and 16, which portions having
proportions out of order for illustration purposes.
FIGS. 5a and 5b show an electroactive multilayer composite 15
having the first composite 1 arranged with its front surface 20
facing the rear surface 21 of the adjacent composite 1, in the
following referred to in general as a Front-to-Back multilayer
composite 15. In this type of lamination process, the electrically
conductive layer of the first composite 1 is in direct contact with
the rear surface of the second composite 1. The composites 1 are
laminated either by the use of an elastomer of the same type as
used for producing the dielectric film 2 or alternatively, the two
composites 1 are stacked without use of an adhesive. For some
purposes it is preferred that the multilayer composite is made of
stacked composites without the use of an adhesive. In these cases,
the wave troughs are simply filled with air.
Due to the pattern of raised and depressed surface portions 3, the
electrically conductive layer of each of the composites may even
out as the film expands, and recover its original shape as the film
contracts along the direction defined by arrows 5 (see FIG. 5b)
without causing damage to the electrically conductive layers, this
direction thereby defining a direction of compliance. Thus, the
multilayer composite 15 shown in FIG. 5b is designed to be very
compliant in the direction defined by arrows 5 and designed to be
very stiff in the transverse direction defined by arrows 6.
FIGS. 5c and 5d illustrate the electroactive multilayer composite
15 being exposed to zero electrical potential difference and being
exposed to a high electrical potential difference. As can be seen
from FIG. 5d the dielectric film is expanded, while the
electrically conductive layers are evened out, when exposed to an
electrical potential difference. It can further be seen that the
depth of the wave troughs (the corrugation depth d) is reduced when
the multilayer composite is exposed to an electrical potential
difference. The composites can be bonded by applying a high
electrical potential difference to the stacked composites, whereby
the film of one composite and the electrically conductive layer of
an adjacent composite adhere to each other without the use of an
additional adhesive. Thus, they may be brought into intimate
contact by electrostatic forces. Alternatively, they may adhere to
each other by pressing them together, e.g. by the use of rollers,
due to the characteristics of the dielectric film which may be
slightly tacky when made of an elastomer.
As an alternative hereto, FIGS. 6a and 6b show an electroactive
multilayer composite 16 having the first composite 1 arranged with
its rear surface 21 facing the rear surface 21 of the adjacent
composite 1, in the following referred to in general as a
Back-to-Back multilayer composite 16. The composites 1 are
adhesively bonded either by the use of an elastomer adhesive with
characteristics similar to the dielectric film 2 of the composites
1. Alternatively, the two composites 1 are stacked without use of
an adhesive.
In the electroactive multilayer composite 16 illustrated in FIG.
6a, the corrugated surfaces 3 can be coated with the electrically
conductive layer before or after laminating the composites 1. The
Back-to-Back multilayer composite 16 has the advantage that the
impact of defects in the dielectric film 2, pin-holes in the
electrically conductive layer etc. may become less critical if the
adjacent layer does not have similar errors in close vicinity.
If the individual composites 1 are made in identical production
steps, there may be an increased risk that identical errors exist
on the same location of each composite 1. To reduce the impact of
such errors, it may be an advantage to shift the location of one
composite 1 relative to an adjacent composite 1, or to rotate the
composites 1 relative to each other.
The lamination process represents a critical step in the production
process. Thus, precise lamination machines equipped with tension
control are to be used.
Similar to the multilayer composite 15 the multilayer composite 16
shown in FIG. 6b is designed to be very compliant in the direction
defined by arrows 5 and designed to be very stiff in the transverse
direction defined by arrows 6.
FIGS. 6c and 6d illustrate the electroactive multilayer composite
16 being exposed to zero electrical potential difference and being
exposed to a high electrical potential difference. As can be seen
from FIG. 6d the dielectric film is expanded, while the
electrically conductive layers are evened out, when exposed to an
electrical potential difference.
FIG. 7a illustrates that an electroactive multilayer composite 15
of the kind illustrated in FIG. 5a may further contain an endless
number of composites 1 depending on the specific need. The
multilayer composite in FIG. 5a contains one dielectric film 2 out
of two dielectric films 2 which is inactive, i.e. only one of the
two dielectric films 2 is located between two electrically
conductive layers (not shown). FIG. 7a illustrates that a larger
number of composites decreases the impact of the inactive layers on
the electroactive multilayer composite 15 as such, since all but
the lowermost composite 15 are located between electrodes.
FIG. 7b illustrates an alternative way of forming an electroactive
multilayer structure 15 containing an endless number of composites
1. The composites 1 have been laminated by means of adhesive layers
22 arranged between the composites 1 in such a manner that the
composites 1 are not in direct contact with each other. The
material of the adhesive layers 22 has properties similar to those
of the dielectric material of the composites 1, in terms of ability
to stretch. This is in order to allow the adhesive layers 22 to
stretch along with the dielectric material when the multilayer
structure 15 is working. Thus, the adhesive layers 22 may
advantageously be made from an elastomer, or from a material with
elastomer-like properties.
In FIG. 8, two electroactive multilayer composite 16 of the kind
also shown in FIG. 6a, i.e. Back-to-Back composites, are stacked on
top of each other. In this electroactive multilayer composite, the
electrically conductive layers are pair-wise in contact with each
other. Two dielectric films 2 are located between two of such sets
of two electrically conductive layers. The laminate offers a
reduced impact of production defects in the individual layers.
Furthermore, it is illustrated that a third or even further
electroactive multilayer composite(s) 16 may be added to this
multilayer composite.
FIG. 9 illustrates a stack of multilayer composites 16 similar to
the stack shown in FIG. 8. However, in the situation illustrated in
FIG. 9, the Back-to-Back multilayer composites 16 are stacked
pair-wise, and the pair-wise stacked multilayer composites 16 are
then stacked together. In the stack illustrated in FIG. 9 it is
ensured that the electrically conductive layers of adjacent
pair-wise stacks facing each other has the same polarity.
Accordingly, such a stack can be rolled without risking
short-circuiting of the electrodes, and the stack is therefore
suitable for being rolled, e.g. to form a tubular transducer.
FIG. 10a illustrates a Front-to-Back electroactive multilayer
composite 15 as shown in FIG. 5a being rolled. Since the composite
1 may be produced in very long lengths, so called "endless"
composites, the multilayer composite 15 may also be produced in
very long lengths, thereby allowing for the producing for rolled
multilayer composites comprising numerous windings.
FIG. 10b illustrates rolling of a multilayer composite 15 around
rods 23. The rods 23 are positioned at an end of the multilayer
composite 15, and the composite 15 is then rolled around the rods
23 as indicated. Thereby the multilayer composite 15 obtains a
rolled tubular shape.
FIGS. 11a and 11b show a portion of a composite 24 which is
suitable for forming a rolled or otherwise laminated transducer.
The composite 24 comprises a film 2 made of a dielectric material
having a surface provided with a pattern of raised and depressed
surface portions, thereby forming a designed corrugated profile of
the surface, i.e. the film 2 is similar to the film 2 of the
composite 1 of FIG. 1c. In this case the film 2 is provided with an
electrically conductive layer comprising negative electrode
portions 25 and positive electrode portions 26 arranged in an
interleaved pattern, i.e. the negative electrode portions 25 and
the positive electrode portions 26 appear alternating with a gap in
between. In the gap an electrically conductive layer is not
deposited on the dielectric film. The arrow 27 indicates that the
composite 24 may be a very long, an "endless", composite as shown
in FIG. 13a, and as a folded composite as shown in FIG. 13b.
FIGS. 12a-12c illustrate one possible method of making the
composite 24 of FIG. 11. FIG. 12a illustrates the film 2 being a
very long film on two rolls 30. The electrically conductive layer
(not shown) is deposited on the film 2 using a non-continuous
vapour deposition roll to roll method. The arrows 31 indicate the
process direction. The electrically conductive layer is deposited
through a shadow mask 32 in order to provide gaps in between the
electrode portions 25, 26. When the electrically conductive layer
is deposited on an area of the film 2, the film 2 is rolled in the
direction of the arrows 31 and stopped. A shutter (not shown) is
opened and the electrically conductive layer is deposited on the
next area of the film 2, this area being adjacent to the previous
area, and ensuring a continuous transition contact between
electrodes with the same polarity. The shutter is closed when the
required thickness of the electrically conductive layer is
achieved. The electrode deposition principle where electrodes are
deposited through a shadow mask is, for practical reasons, more
appropriate for production of electrodes with constant width and
gap. As an alternative, the gap may be made by means of laser
ablation. In fact, it is preferred to make the gap by means of
laser ablation, since when using such a technique it is very easy
to provide a variable distance between each gap and thus a variable
width of each portion of the electrically conductive layer. This
will be explained in further detail below.
FIG. 13a illustrates the composite 24a of FIG. 11a and FIGS.
12a-12b formed as a rolled composite 35. D and R denote diameter
and radius of a roll 36 onto which the composite 24 is rolled. The
solid lines denote positive electrodes while the dotted lines
denote negative electrodes. It should be noted that for the sake of
clarity, the rolled composite is shown by means of concentric
circles. However, it should be understood that in reality the
rolled composite forms a spiral pattern. The width, w, of the
electrode portions 25 and 26 and the width of the gap between these
electrode portions are determined based on the cross section of the
roll 36 as follows: 2.pi.(R)=w+gap, where the gap is very small as
compared to w. Furthermore, it is preferred that the thickness t of
the composite 24a is smaller than the gap. Otherwise, the
efficiency of the transducer which is formed by this roll process
becomes low. When a winding n is made by rolling the composite 24a,
the gap is tangentially shifted by a film thickness order, 2.pi.tn
with respect to the previous winding. Thus if the gap shift exceeds
the gap width, electrodes with same polarity will tend to overlap,
and this renders the corresponding portions of the capacitor
inactive. This method is preferred for building actuators with
limited number of windings and operating in a pre-strained
configuration or flat tubular actuator configurations where
electrode portions and gaps are deposited in the portions of
dielectric web that correspond to flat portions of the flat tubular
actuator. An alternative method where laser ablation is used to
design the electrodes with variable width but constant gap width is
more appropriate for the rolled tubular actuator. In this case, the
width of the gap and depleted regions is determined by the
traveling laser spot size, and the width of a given electrode
associated to a given winding of the growing circumference of the
actuator is such that width and gap match the winding
circumference.
Similarly, FIG. 13b illustrates the composite 24b of FIG. 11b as a
folded composite 37. It is clear from FIG. 13b that the composite
24b is folded carefully in such a manner that it is ensured that
electrodes 25, 26 of opposite polarity do not come into direct
contact.
FIGS. 14a and 14b illustrate lamination of the composite shown FIG.
11 by folding of the composite 24. Alternatively, the composite
could be of the kind shown in FIGS. 1a and 2. The composite 1, 24
is manufactured in a long structure, thereby defining a length and
a width of the composite 1, 24, and has a surface 3 with a pattern
of raised and depressed surface portions. The pattern defines waves
of crests and troughs, extending in a common direction, and the
common direction is arranged substantially along the width of the
long structure. Accordingly, the composite 1, 24 is compliant in a
direction perpendicular to the common direction, i.e. along the
length of the long structure.
The composite 1, 24 of FIG. 14a is laminated by folding the long
structure along the length, i.e. in such a manner that the width of
the resulting electroactive multilayer composite 40 is identical to
the width of the composite 1, 24. Due to the orientation of the
compliant direction of the composite 1, 24 the electroactive
multilayer composite 40 will be compliant in a direction indicated
by arrows 41.
FIG. 14b illustrates lamination of a composite 1, 24 according to
another embodiment of the invention. This is very similar to the
embodiment shown in FIG. 14a. However, in this case the common
direction is arranged substantially along the length of the long
structure, and the composite 1, 24 is therefore compliant in a
direction along the width of the long structure, as the composite
of FIG. 1b. Accordingly, the resulting electroactive laminate 42
will be compliant in a direction indicated by arrows 43.
Thus, the laminated composite shown in FIG. 14a is compliant along
the length of the laminated composite. This means that the
structure of FIG. 14a can be made to be of any length, and thus of
any desired stroke length. Similarly, the laminated composite of
FIG. 14b is compliant along the width of the laminated composite.
This means that the structure of FIG. 14b can be made to be of any
width. Thus, it is possible to design a transducer with any
appropriate dimensions in accordance with geometrical requirements
of the intended application.
FIGS. 15a-15c are perspective views of direct actuating transducers
50 according to embodiments of the invention. The direct actuating
transducer 50 of FIGS. 15a-15c have been manufactured by rolling a
multilayer composite, e.g. of the kind shown in FIG. 1a or in FIG.
5. The transducer 50a of FIG. 15a is solid, whereas the transducer
50b of FIG. 15b is hollow. The transducers 50 may have any
elongated form, e.g. substantially cylindrical with a cross section
which is substantially circular, elliptical or curve formed as
illustrated in FIG. 15c.
In FIGS. 15a-15c the composite, which has been rolled to form the
columnar shaped transducers 50, has a direction of compliance which
is parallel to the directions indicated by arrows 51. Accordingly,
when electrical energy is applied to the electrodes of the direct
actuating transducers 50, the transducers 50 will elongate axially
in the direction of the arrows 51. It has now been found that if
the transducers 50 are properly made and dimensioned in accordance
with certain aspects of the invention, they are able to exert
significant force against an axial load which tends to resist the
axial elongation.
As indicated earlier in this specification, the electroactive
composite of the present invention is quite supple and pliable,
resembling ordinary household cling film or polyethylene shopping
bag sheet material in pliability. The composite differs from those
materials by its higher elasticity and its mechanical anisotropy,
as previously explained, being very stretchy in one direction and
much less stretchy in the perpendicular direction.
The inventors now have realised that despite of the suppleness,
pliability and elasticity of the composite, a roll formed by
winding up a sufficient length of the composite will be quite
stiff. If the roll is properly wound with respect to the mechanical
anisotropy of the film, it will have axial compliance brought about
by the mechanical anisotropy, and yet it can be quite resistant to
buckling under axial load.
Accordingly, a composite of corrugated anisotropic dielectric film
layers with electrically conductive electrode layers can be rolled
into a tubular shape with a number of windings sufficient to make
the resulting structure of the tubular element sufficiently stiff
to avoid buckling. In the present context, the term `buckling`
means a situation where an elongated structure deforms by bending
due to an applied axial load. It has been found that no additional
component such as any stiffening rod or spring inside the elongated
structure is necessary to obtain sufficient stiffness to avoid
buckling under technically useful levels of axial load. The
required stiffness is obtained merely by winding up a sufficient
number of windings of the composite material.
The rolled structures illustrated in FIGS. 15a-15c are designed to
withstand a specified maximum level of load at which the stiffness
is sufficient to avoid buckling. This specified maximum level may,
e.g., be a certain level of force at a certain level of elongation,
or it may be a maximum level of actuation force, a blocking force,
or a higher level of force occurring when the transducer is
compressed to a shorter length against the direction of the arrows
51.
Design parameters for the direct actuating transducer as described
in the present application are optimised according to design rules
developed by the inventors. These design rules allow for
determining the optimum dimensions of a rolled actuator
(transducer) based on the actuator performance specifications.
The mechanical and electrostatic properties of an electroactive
composite are used as a basis to estimate actuator force per unit
area and stroke. Rolled actuators as described in accordance with
the present invention are made by rolling/spooling very thin
electro-active composites, e.g. as shown in FIGS. 1a and 1b, having
a thickness in the micrometers range. A typical actuator of this
type can be made of thousands of windings and can contain as many
as 100 windings per millimeter of actuator wall thickness.
When activated, direct/push actuators convert electrical energy
into mechanical energy. Part of this energy is stored in the form
of potential energy in the actuator material and is available again
for use when the actuator is discharged. The remaining part of
mechanical energy is effectively available for actuation. Complete
conversion of this remaining part of the mechanical energy into
actuation energy is only possible if the actuator structure is not
mechanically unstable, like the well-known buckling mode of failure
due to axial compression. This can be achieved by properly
dimensioning the cross-sectional area of the actuator in relation
to actuator length. Mathematically this corresponds to Euler's
theory of column stability; in accordance with the invention, this
theory also applies to an actuator column formed by rolling up a
sufficient number of windings of electroactive multilayer
composite.
The optimisation process starts by defining the level of force
required for a given application. Then based on the actuator force
per unit area, it is possible to estimate the necessary cross
sectional area to reach that level of force.
For a cylindrical structure, the critical axial load or force Fc
for a given ratio between length and radius of the cylinder is
given by:
.pi. ##EQU00001## where c is a boundary condition dependent
constant, E is the modulus of elasticity, A is the cross sectional
area of the cylinder, L is the length of the cylinder, and R is the
radius of the cylinder.
Consider now an electro-active polymer transducer of cylindrical
shape which is actuated by applying a voltage, V, to its
electrodes. In the unloaded state, the transducer will simply
elongate. If restrained by an axial load, the transducer will exert
a force upon the load which increases with the voltage, V. The
maximum force, F.sub.max, which the transducer can be actuated to
depends on the construction of the transducer.
For a given length L and cross section A, this means that the
voltage needs to be controlled in such a manner that forces higher
than F.sub.max<F.sub.C are not allowed. For a given cross
section, this means that the length of the cylinder must be smaller
than a critical length, L.sub.C, i.e. L<L.sub.C, with L.sub.C
defined as follows.
For a transducer 50 with a given cross section and a chosen maximum
force level, the maximum force level being related to the maximum
voltage level, the critical length, L.sub.C, can be derived from
the formula:
.pi. ##EQU00002## and the design criteria is L<L.sub.C.
For a selected voltage level a transducer 50 with a given cross
section is able to actuate with a given maximum force, the
so-called blocking force, F.sub.bl, at 0% elongation. In this
situation the design criterion is:
.pi. ##EQU00003##
Applying these design criteria for a transducer 50 made of an
elastomer with E=1 MPa, F.sub.bl/A=20 N/cm.sup.2 and c=2, the
design rule for F.sub.max=F.sub.bl will be L.sub.bl=10r, i.e. the
so-called slenderness ratio, .lamda., must fulfil the following
condition in order to obtain a non-buckling structure at the load
being equal to the blocking force: .lamda..gtoreq.L/r=10.
For alternatively chosen lower levels for the actuating force for
the same transducer 50, i.e. for a cylindrically symmetric
transducer 50 with the same radius, r, the design criteria for
length L can be derived from the following formula:
L.gtoreq.L.sub.bl {square root over (F.sub.bl/F)}.
This may, e.g., mean that if the actuation level at 10% elongation
is 1/4F.sub.bl, then the length, L, of that transducer at 10%
elongation is: L.gtoreq.L.sub.bl {square root over
(1/1/4)}=L.sub.bl2.
The theory of Euler can be applied to designing a transducer 50
with a specific need for transducer stroke and a chosen percentage
of elongation of the dielectric film. Since there is no limitation
to increase in cross sectional area, A, of the cylindrical
symmetric transducer 50a and 50b due to an increased number of
windings, and because the design rules derived from the theory of
Euler are fulfilled, it is possible to simply provide the necessary
number of windings to obtain a required level of actuation force.
Accordingly, the technology described above makes it possible to
build dielectric transducers having non-buckling characteristics at
a given force level and a given stroke for direct actuation.
When designing a direct acting capacitive transducer, it is
necessary to dimension its mechanical structure against buckling.
This is done typically by increasing the area moment of inertia of
its cross section, known as I. As an example, a piece of paper with
a given thickness (h), width (w) and length (L) will bend when a
little force is applied to the paper in a direction parallel to its
length. However, by rolling it in the width direction, a much
larger force will be necessary to make it buckle. Rolled-to-flat
bending stiffness ratio is then given by
.pi. ##EQU00004## An example of such is to take w=40 mm and h=1 mm,
then the ratio is about 245.
Stabilisation of the actuator against any mechanical instability
requires dimensioning its cross section by increasing its area
moment of inertia of the cross section I. Low values of I result in
less stable structures and high values of I result in very stable
structures against buckling. The design parameter for dimensioning
the structure is the radius of gyration r.sub.g which relates cross
section A and area moment I. Low values of r.sub.g result in less
stable actuator structures and high values of r.sub.g result in
very stable actuator structures. After having defined optimum
ranges for both area A and radius of gyration r.sub.g, it is
possible to define the optimum range for the rolled actuator wall
thickness, t, with respect to r.sub.g in the form of t/r.sub.g.
Area A, radius r.sub.g and wall thickness t are the design
parameters for dimensioning the actuator cross-section for maximum
stability. Low values of t/r.sub.g result in very stable actuator
structures and high values of t/r.sub.g result in less stable
actuator structures.
Once the ranges of the cross section parameters have been
determined, it is necessary to estimate the maximum length L of the
actuator for which buckling by axial compression does not occur for
the required level of force. Slenderness ratio defined as the
length L to radius of gyration r.sub.g ratio is the commonly used
parameter in relation with Euler's theory. Low values of L/r.sub.g
result in very stable actuator structures and high values of
L/r.sub.g result in less stable actuator structures against
buckling.
Once all design parameters for the optimum working direct actuator
have been determined, it is possible to estimate the total number
of windings that are necessary to build the actuator based on the
actuator wall thickness t and the number of windings per millimeter
n for a given electro-active composite with a specific thickness in
the micrometer range.
In a preferred embodiment, the ratio between the number n of
windings and the wall thickness t of the transducer, n/t, should be
in the range of 10 windings/mm-50 windings/mm. Furthermore, the
slenderness ratio, being the ratio between the length L of the
transducer and the gyration radius r.sub.g of the transducer should
be less than 20. The gyration radius r.sub.g is defined as r.sub.g=
{square root over (I/A)}, where I is the area moment of a cross
section and A is the cross sectional area of the transducer.
Thus, by carefully designing transducers in accordance with the
present invention, it is possible to obtain large actuation forces,
even though a very soft dielectric material is used. Actuation
forces may even reach levels comparable to conventional transducers
made from harder materials, e.g. magnetic transducers. This is a
great advantage.
FIG. 16a is a graph illustrating force as a function of stroke in a
direct actuating transducer according to an embodiment of the
invention. When voltage is applied to the anisotropic compliant
electrically conductive layers of the transducer, electric field
induced compression across film thickness is converted into
elongation/stroke along the compliant direction of the transducer.
The corresponding stress is referred to as Maxwell stress, P, and
the corresponding actuation force is referred to as electrostatic
force F.sub.electrostatic. Upon elongation, the dielectric material
exerts a counterforce F.sub.elastomer which increases with
transducer stroke as shown in FIG. 16a.
Consequently, the effective force available for direct actuation
F.sub.act is a result of the two described forces, and
F.sub.act=F.sub.electrostatic-F.sub.elastomer, as shown in FIG.
16b. The characteristic curve representing force versus stroke of
the direct actuating transducer is typical for force transducers,
where actuation force decreases as a function of increasing stroke,
until a maximum value of the stroke is reached corresponding to
"zero" actuation force as depicted in FIG. 16b.
FIG. 16c illustrates the range of calculated direct actuation
forces as a function of transducer stroke for different outer
diameters of a direct acting capacitive transducer, a rolled
transducer. Large actuation forces in the range of hundreds to
thousands of Newtons can be generated. Blocking forces are
typically 4 orders of magnitude larger than nominal actuations
forces defined at 10% transducer stroke. A direct acting capacitive
transducer made of a 40 micrometer thick dielectric material with
elastic modulus in the range of 0.5-1 MPa will generate a force per
unit area in the range of 0.1-0.2 N/mm.sup.2, for a typical
actuation voltage of 3000 volts. When considering large transducer
cross sections, this corresponds to large actuation forces as shown
in FIG. 16c.
FIGS. 17a and 17b are perspective views of direct actuating
transducers 52 according to alternative embodiments of the
invention. The transducers 52 of FIGS. 17a and 17b have a direction
of compliance along the tangent of the cylinder. Accordingly, the
elongation of the transducers 52 takes place on a perimeter of the
tubular structure, illustrated by the arrows 53, i.e. the
transducer 52 is caused to expand and contract in a radial
direction.
FIG. 18a illustrates lamination of a composite 1 to form a flat
tubular structure 60. The composite 1 may advantageously be of the
kind shown in FIGS. 1a and 2. The transducer 60 is a laminate of a
sufficiently high number of adhesively bonded composites to ensure
a rigidity of the transducer, which rigidity is sufficient to
enable that the transducer can work as an actuator without being
pre-strained. The transducer 60 is manufactured by winding a
continuous composite, e.g. of the kind shown in FIGS. 1a and 2, in
a very flat tubular structure. Using this design the limitations
regarding number of layers described above are eliminated. Thereby,
the transducer 60 can be made as powerful as necessary, similarly
to what is described above with reference to FIGS. 15a-15c.
The flat tubular structure of the transducer 60 shown in FIG. 18a
is obtained by rolling the composite 1 around two spaced apart rods
61 to form a coiled pattern of composite 1. Due to the orientation
of the compliant direction of the composite 1, the flat tubular
structure 60 will be compliant in a direction indicated by arrows
62. FIG. 18b illustrate the transducer of FIG. 18a being
pre-strained by two springs 63.
FIGS. 19a-19c are perspective views of transducers 70 having a flat
structure. The transducer 70 is a multilayer composite of a
sufficiently high number of adhesively bonded composites to ensure
a rigidity of the transducer, which rigidity is sufficient to
enable that the transducer can work as an actuator without being
pre-strained. The transducer 70 is manufactured by laminating a
continuous composite, e.g. of the kind shown in FIGS. 1a and 2, in
a flat structure. Using this design the limitations regarding
number of layers described above are eliminated. Thereby, the
transducer 70 can be made as powerful as necessary, similarly to
what is described above with reference to FIGS. 15a-15c. The
transducer 70a is a multilayer composite of a sufficiently high
number of adhesively bonded composites to ensure a rigidity of the
transducer, which rigidity is sufficient to enable that the
transducer can work as an actuator without being pre-strained. The
transducer 70b is dimensioned by stacking a number of transducers
70a. As an alternative hereto, the transducer 70c may be
pre-strained by a spring 71 or by other elastically deformable
elements.
The transducer 70a and 70b is provided with fixation flanges 72 in
order to attach the transducer in an application, e.g. in order for
the transducer to work as an actuator. The arrows 73 indicate the
direction of compliance.
FIGS. 20a-20e illustrate actuating transducers 80 provided with a
preload. FIG. 20a is a perspective view of a flat transducer 80
provided with fixation flanges 81. The flat transducer 80 of FIG.
20a is pre-strained by a spring 82. Accordingly, the flat
transducer 80 has a direction of actuation indicated by arrows 83.
FIG. 20b illustrates a similar flat transducer 80 in which the
spring is replaced by a similar second flat transducer 80. FIG. 20c
illustrates half of a transducer, the transducer being similar to
the transducer of FIG. 20b and dimensioned by the use of a number
of identical transducers (only half of them are shown). FIGS. 20d
and 20e illustrate two alternative transducers 84 and 85 each
comprising a number of flat transducers 80 being pre-strained by
adjacent transducers similar to the transducer of FIG. 18b. The
transducers 84 and 85 actuate cross directional, in FIG. 20d in a
carpet-like structure and in FIG. 20e in a wall-like structure.
It should be noted that the transducers of FIGS. 18-20 only require
pre-strain along one direction, i.e. in the direction of
compliance. Thus, a pre-strain in a direction transverse to the
direction of compliance, which is necessary in prior art
transducers, is not required in transducers according to the
present invention.
FIG. 21a illustrates two pre-strained transducers 90 having a flat
tubular structure, the transducers 90 actuating in the longitudinal
direction and thereby rotating an actuating shaft 91.
FIG. 21b illustrates two mechanically pre-strained flat transducers
92, 93 provided with mechanical connection 94, which is supported
by a guiding element for sliding purposes. The transducers 92, 93
are shown in three situations. In the first situation neither of
the transducers 92, 93 are active. However, they are both
mechanically pre-strained. In the second situation, transducer 93
is active. Since the transducer 92 is inactive, the transducer 93
causes transducer 92 to relax, thereby releasing some of the
mechanical pre-strain of transducer 92. In the third situation
transducer 92 is active while transducer 93 is inactive. Transducer
92 thereby causes transducer 93 to relax, thereby releasing some of
the mechanical pre-strain of transducer 93. Thus, the transducers
92, 93 in combination with the mechanical connection 94 form a
double-acting transducer in which one of the transducers causes the
other transducer to relax and release mechanical pre-strain.
FIG. 22 illustrates an electroactive composite comprising a
dielectric film 2 with a first surface 100 and a second surface 101
being opposite to the first surface 100. Both surfaces of the
dielectric film 2 are partly covered with an electrically
conductive layer. Due to the shape and location of the electrically
conductive layers, an active portion A exists, in which electrode
portions 102, 103 of the electrically conductive layers cover both
surfaces 100, 101 of the dielectric film 2. The electrically
conductive layers further define a first passive portion B in which
only the second surface 101 of the dielectric film 2 is covered by
a contact portion 104 of one of the conductive layers and a second
passive portion C in which only the first surface 100 of the
dielectric film 2 is covered by a contact portion 105 of the other
conductive layer. As it appears, the electroactive composite can be
electrically connected to a power supply or connected to control
means for controlling actuation of the composite by bonding
conductors to the contact portions 104, 105. Even if the
illustrated composite is laminated, rolled, or folded to form a
transducer with a large number of layers, the electrode portions
102, 103 may easily be connected to a power supply e.g. by
penetrating the layers in each contact portion 104, 105 with an
electrically conductive wire or rod and by connecting the wire or
rod to the power supply. The ratio between the thickness of the
dielectric film 2 and the thickness of the electrically conductive
layers is merely for illustration purposes. The process illustrated
in FIG. 22 may be referred to as `off-set`, since the contact
portions 104, 105 are provided by applying the electrode portions
102, 103 on the surfaces 100, 101 of the dielectric film 2
`off-set` relatively to each other.
FIGS. 23a-23c illustrate three different ways of space shifting two
composites 1 of a multilayer composite forming a transducer where
each composite 1 comprises an electrically conductive layer on a
dielectric film. The illustrated composites 1 have a compliance
direction in which they expand or contract when the transducer is
activated. In FIG. 23a, the contact portions are space shifted
along the compliance direction, in FIG. 23b, the contact portions
are space shifted perpendicular to the compliance direction, and in
FIG. 23c, the contact portions are space shifted both in the
compliance direction and in a direction being perpendicular to the
compliance direction. In any of the configurations, it is desired
to keep the region where the physical contact is made between the
multilayer composite and the connecting wire, rod or similar
conductor away from any source of stress or moving parts. FIG. 23d
illustrates the multilayer composite in a side view.
Thus, FIGS. 22 and 23a-23c illustrate two different principles for
providing contact portions 104, 105, i.e. the `off-set` principle
in FIG. 22 and the `space shifting` principle in FIGS. 23a-23c.
These principles may be combined with various lamination processes,
and a principle which is appropriate for the intended application
may accordingly be chosen.
FIG. 24 illustrates that contact portions 104, 105 form part of
electrically conductive layers and form extension islands on one
side of the electrode portions 102 and 103. The islands of two
adjacent composites in a multilayer composite are located
differently so that the contact portions 104, 105 of adjacent
composites are distant from each other.
FIG. 25 illustrates two composites each provided with an
electrically conductive layer. When the composites are joined in a
multilayer structure, they are offset relative to each other so
that a portion of the electrically conductive layer on each
composite forms a contact portion 104 being distant from the
corresponding contact portion 105 on the other composite.
FIGS. 26 and 27 illustrate tubular transducers 50.as shown also in
FIGS. 15a and 15b. The tubular transducers are connected to a power
supply at the indicated contact portions 104, 105.
FIG. 28 illustrates a transducer 110 with a flat tubular structure.
The transducer comprises contact portions 104, 105 on an inner
surface. The contact portions may be connected to a power supply
e.g. via one of the elongated rods 111 with electrically conductive
contact portions. The rod 111 is shown in an enlarged view in FIG.
29 in which it can be seen that the rod 111 comprises two contact
portions 112, 113 which come into contact with the contact portions
104, 105 of the flat tubular structure when the rod 111 is inserted
into the tubular structure. The rods 111 could form part of a
device on which the transducer operates. Both space-shifted and
off-set electrode principles can be applied in contacting the above
described transducer structure.
FIG. 30 shows three different kinds of connectors, i.e. a soft
connector 120, a metal coated plastic connector 121, and a metal or
metal coated grid strip connector 122. The soft connector 120
comprises an elastomer film 123 coated with a layer of electrically
conductive material 124. Similarly, the metal coated plastic
connector 121 comprises a plastic portion 125 coated with a metal
layer 126.
FIGS. 31-35 illustrate composites 1 provided with electrical
contacts. Since the composite 1 of the present invention is very
soft, it is a challenge to join the composite 1 to a somewhat
stiffer normal electrical connector, such as a wire, a strip, a
grid, etc.
FIG. 31 shows a soft connector 120 connected to a composite 1
comprising a dielectric film 2 with a corrugated surface 3 provided
with a layer of electrically conductive material 4. The
electrically conductive parts 124, 4 of the soft connector 120 and
the composite 1, respectively, have been joined via a layer of
electrically conductive adhesive 127, thereby electrically
connecting the composite 1 and the soft connector 120.
FIG. 32 shows two composites 1 having been joined as described
above, i.e. via a layer of electrically conductive adhesive 127,
and the composite 1 positioned on top is used as main electrode to
a power supply.
FIG. 33 shows a metal or metal coated wire or strip 128 connected
to a composite 1. The metal or metal coated wire or strip 128 is
adapted to be connected to a main power supply. Similarly to what
is described above, the metal or metal coated wire or strip 128 is
joined to the electrically conductive layer 4 of the composite 1 by
means of an electrically conductive adhesive 127. However, in this
case the electrically conductive adhesive 127 is arranged in such a
manner that it surrounds a periphery of the metal or metal coated
wire or strip 128, thereby providing a very efficient electrical
contact between the metal or metal coated wire or strip 128 and the
electrically conductive layer 4 of the composite 1.
FIG. 34 shows a metal or metal coated grid strip connector 122
connected to a composite 1 via an electrically conductive adhesive
127. As described above with reference to FIG. 33, the electrically
conductive adhesive 127 is arranged in such a manner that a part of
the metal or metal coated grid strip connector 122 is completely
surrounded, thereby providing a very good electrical contact.
FIG. 35 shows a metal coated plastic connector 121 connected to a
composite 1 via a layer of electrically conductive adhesive 127. As
described above with reference to FIGS. 31 and 32, the layer of
electrically conductive adhesive 127 is arranged between the metal
layer 126 of the metal coated plastic connector 121 and the
electrically conductive layer 4 of the composite 1, thereby
providing electrical contact there between.
FIG. 36a illustrates the process of manufacturing a tool or mould
for the process of making the composite, e.g. a composite 1 as
illustrated in FIG. 1. FIG. 36b illustrates the process of
manufacturing the composite by use of the tool, and FIG. 36c
illustrates the process of making a transducer from the
composite.
Thus, we start the process by making a master mould having the
desired corrugation profile. We may fabricate the mould by laser
interference lithography on photoresist coated glass, or by
standard photolithography on silicon wafers.
For the standard photolithography on silicon wafers, the exposure
mask is relatively simple and may preferably exhibit equally spaced
and parallel lines, e.g. having a width of 5 .mu.m and a spacing of
5 .mu.m. Standard silicon micromachining recipes are then used to
etch the silicon in order to form so-called V-grooves, i.e. grooves
having a cross sectional shape resembling a `V`. A series of
oxidation and hydrofluoric acid etching steps are then performed to
transform the V-grooved structures into quasi-sinusoidal
corrugations, if this is the desired shape.
We can fabricate master moulds of a relatively large size, such as
up to 32 cm.times.32 cm, by means of laser interference
lithography. In laser interference lithography two laser beams,
each with an expanded spot size and with uniform energy
distribution across the beam cross section, are caused to interfere
onto a photoresist coated glass substrate. Such a process does not
require any exposure mask, and relies on the interference
phenomenon known in the field of optics. The result of exposure,
development and, finally, hard-baking, is a direct sinus waveform
profile written onto the photoresist, where profile period and
amplitude are determined by the laser beam wavelength, the
incidence angles of the laser beams onto the photoresist, and the
thickness of the photoresist.
In the next step of the process as illustrated in FIG. 36, we use
standard stress-free electroplating processes to fabricate a
sufficient number of nickel copies or moulds necessary in order to
obtain replication of corrugated microstructures onto plastic
rolls. These nickel replicas also called shims have a thickness in
the 100 micrometer range. These shims are mechanically attached in
a serial configuration to form a "belt" having a total length which
is precisely set to match the circumference of the embossing drum.
Use of thin shims facilitates bending them without building too
much stress and subsequently rolling the "belt" around the drum
circumference. Each shim is placed with respect to its neighbours
in such a way that corrugation lines are adjusted with micrometer
accuracy for minimising any angular misalignment between lines of
neighbouring shims. Then the corrugated microstructures of the
embossing drums, resulting from the nickel moulds, are accurately
replicated onto plastic rolls. We may do so by means of
roll-to-roll micro embossing (UV or heat curing). Roll-to-roll
embossing allows for the production of rolls of micro-embossed
plastic material having lengths in the range of hundreds of meters.
We use the micro-embossed plastic rolls as carrier web, e.g. in the
form of a belt or a mould, for the production of dielectric films
having single-surface or double-surface corrugations, e.g.
elastomer films having lengths in the range of hundreds of
meters.
We fabricate corrugated elastomer films or sheets of limited size
by well known spin coating. It is a discontinuous process, and the
maximum size of the film or sheet is determined by the size of the
mould. Alternative types of production processes are the kinds
developed for the polymer industry, such as adhesive tapes,
painting, etc., normally referred to as `roll-to-roll coating` or
`web coating`. These production processes are large scale, large
volume, and continuous processes.
In a subsequent step, we fabricate elastomer films using the
micro-embossed plastic roll, e.g. using a roll-to-roll, reverse
roll, gravure, slot die, bead or any other suitable kind of coating
technique. As a result an elastomer coated plastic film is
obtained. To this end reverse roll and gravure roll coating
techniques are considered the most promising among other known
techniques because they offer coatings which are uniform and have a
relatively well defined thickness. We select the surface properties
of the embossed plastic roll or mould and of the embossing resin in
a manner which allows for wetting by the elastomer material. We
carry out the production process of the elastomer film in a clean
room environment in order to fabricate pinhole-free elastomer films
of high quality.
We expose non-cured elastomer film formed onto the mould as
described above to heat, ultraviolet light or any other source
capable of initiating cross-linking, in order to cause the
elastomer film to cure. The chosen source will depend on the type
of elastomer material used, in particular on the curing mechanism
of the used material.
Then we release the cured film from the mould in a delamination
process. To this end appropriate release tooling is used. We may
preferably choose mould material and elastomer material to
facilitate the releasing process. Very weak adhesion of cured
elastomer to the substrate mould is preferred. If very good
adhesion occurs, the release process can fail and damage the film.
A single-sided corrugated elastomer film roll is the product of
this delamination process.
In the next step we deposit the metal electrode onto the corrugated
surface of the elastomer film by means of vacuum web metallization.
Accordingly, a metal coating, e.g. a coating of silver, nickel,
gold, etc., is applied to the corrugated surface. Thus, a composite
is formed.
The challenge in the large scale manufacturing of elastomer film
having lengths in the range of kilometers is not in the production
of flat films, but rather in the production of single-sided or
double-sided corrugated film with precise and very well defined
micro structures. Another challenge is in handling these very soft
materials using controlled tension forces which are several orders
of magnitude smaller than the control tension forces normally
occurring in the polymer industry. Metallization of a corrugated
elastomer film surface with reliable coating layers, when the
thickness of a coating layer is only 1/100 of the depth of the
corrugated pattern, is yet another challenging issue of the
production process.
Next, we laminate the coated elastomer films, the composites,
thereby forming a multilayer composite, as described above. Then we
roll the multilayer composite to form the final rolled transducer
structure. The rolled transducer undergoes finishing and cutting,
and electrical connections are applied.
Finally, we may integrate the finished transducer into a final
product along with control electronics, and the transducer is ready
for use.
While the present invention has been illustrated and described with
respect to a particular embodiment thereof, it should be
appreciated by those of ordinary skill in the art that various
modifications to this invention may be made without departing from
the spirit and scope of the present invention.
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